1. Field of the Invention
The invention relates to a photoreceptor for use in electrophotography and more specifically to an improved photoreceptor having a high sensitivity and a high endurance by incorporation of a specified compound in the electric charge carrier transport layer.
2. Prior Art
Recent development of electrophotographic copying machines and printers is so remarkable that various kinds of machines and printers have been developed, accompanied with the development of many kinds of photoreceptors suitable for use with them.
Until recently inorganic compounds have been mainly used as electrophotographic photoreceptors from the standpoint of sensitivity and endurance. Such inorganic compounds include zinc oxide, cadmium sulfide and selenium. However, most of the inorganic electrophotographic photoreceptors according to the prior art contain health hazardous materials, so that the disposal thereof is a problem and causes environmental pollution. Further, when selenium, excellent in sensitivity, is used, a thin film thereof must be formed on a conductive support by vapor deposition or the like, which brings about a lowering in the productivity and an increase in the cost. Recently, an amorphous silicon photoreceptor has been noted as being a harmless inorganic photoreceptor and further study regarding this photoreceptor is now in progress. However, such an amorphous silicon photoreceptor is disadvantageous in that a thin film of amorphous silicon must be formed, mainly by plasma CVD, so that the productivity thereof is very low and, not only is the material cost high but, also, the running cost is high, although the resulting photoreceptor is excellent in sensitivity.
Meanwhile, an organic photoreceptor has advantages in that it does not cause environmental pollution because of its disposability by fire, such that the formation of a thin film can be carried out by coating in many cases to permit the mass-production of a photoreceptor at a remarkably lowered cost and such a photoreceptor can be fabricated into various shapes, depending upon the use. However, the organic photoreceptor is still a problem both as to in sensitivity and endurance, so that it is necessary to develop a high-sensitivity and high-endurance organic photoreceptor.
Although various methods have been proposed for improving the sensitivity of an organic photoreceptor, a separate type of the photoreceptor having a double-layered structure comprising a generator layer and a transport layer now prevails. For example, electric charges generated by exposure in the generator layer are injected into the transport layer and pass through it to reach the surface of the photoreceptor, where they neutralize the surface charge to form an electrostatic latent image on the surface. This type of photoreceptor is characterized in that the generated charge carriers are trapped in less probability than in a single-layered structure, so that no damage is done to the function of each layer to permit the efficient transport of the charges to the surface (see U.S. Pat. No. 2,803,541).
The organic charge generating agent to be used in the generator layer is selected from compounds which can absorb the energy of radiation to generate electric charges efficiently. Examples of such compounds include azo pigments (see Japanese Patent Laid-Open No. 14967/1979), metallophthalocyanine pigments (see Japanese Patent Laid-Open No. 143346/1985), metal-containing phthalocyanin pigments (see Japanese Patent Laid-Open No. 16538/1975) and squarylium salts (see Japanese Patent Laid-Open No. 27033/1978)
The charge transporting agent to be used in the transport layer must be selected from compounds into which electric charge can be injected from a generator layer with high efficiency and the transport layer is one in which the electric charge can move freely That is, it is suitable to use a compound which has a low ionization potential or generates a radical cation easily. Examples of the compound which has been proposed as the charge transporting agent include triarylamine derivatives (see Japanese Patent Laid-Open No. 47260/1978), hydrazone derivatives (see Japanese Patent Laid-Open No. 101844/1982), oxadiazole derivatives (see Japanese Patent Publication No. 5466/1959), pyrazoline derivatives (see Japanese Patent Publication No. 4188/1977), stilbene derivatives (see Japanese patent publication A No. 198043/1983), triphenylmethane derivatives (see Japanese patent publication B 45-555) and a tristyrylamine (see Japanese patent publication A No. 62-264058).
However, these organic charge transporting agents are inferior to inorganic ones in charge carrier mobility and are unsatisfactory in sensitivity as well.
Since an electrophotographic photoreceptor is exposed to extremely severe conditions in the series of electrophotographic process steps comprising charging, exposure, development, transfer and erasing, the resistance thereof to ozone and abrasion are especially important factors. Therefore, it is necessary that the materials to be used in a photoreceptor be excellent in the resistance. Further, the development of the binder and protective layer to be used in a photoreceptor are also in under investigation. However, no satisfactory photoreceptor has been developed as yet.